EPIDEMICS' TIMING DETERMINED BY LATITUDEGoing back to 1945, Hope-Simpson discovered that influenza epidemics above 30 degrees latitude in both hemispheres occurred during the six months of least solar radiation and that outbreaks in the tropics almost always occured during the rainy season. He thus concluded, "Latitude alone broadly determines the timing of the epidemics in the annual cycle, a relationship that suggests a rather direct effect of some component of solar radiation acting positively or negatively upon the virus, the human host, or their interaction." That is, something may be regularly reducing our immunity every fall and winter. In 2003 researchers confirmed that influenza epidemics in the tropics occur, with few exceptions, during the rainy season, when vitamin D levels should be falling

Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF, Giovannucci E. Epidemic influenza and vitamin D.Epidemiol Infect. 2006 Dec;134(6):1129-40. Epub 2006 Sep 7.PMID: 16959053 doi:10.1017/S0950268806007175

Association Between Serum 25-Hydroxyvitamin D Level and Upper Respiratory Tract Infection. \nAdit A. Ginde, MD, MPH; Jonathan M. Mansbach, MD; Carlos A. Camargo Jr, MD, DrPH . \nArch Intern Med. 2009;169(4):384-390\n

Is sunshine more than just a home remedy for a cold? New research suggests it may be: In a study that will be published tomorrow, people with low levels of vitamin D - also known as the "sunshine vitamin" - were more likely to catch cold and flu than folks with adequate amounts. The effect of the vitamin was strongest in people with asthma and other lung diseases who are predisposed to respiratory infections.